A water tank is a very personal choice but, as you've realised, you have to plan for it before the build.
Options are basically as follows:
Gravity fill the boiler: This is the simplest method of operating the plant and means that once the water in the boiler has been used it has to be vented off, refilled through a funnel and brought up to pressure again. This has the advantage of being a simple plant but means you waste a lot of energy every time you vent off the steam pressure and so uses more gas than necessary, which then generates additional cooling of the gas tank. You can offset these effects by burning liquid from the gas tank, using a larger boiler to give you more capacity and being careful with speed to conserve the water. If you have a very simple plant though and no control over the burner you are forced into going fast enough to use the steam to prevent the safety valve from lifting, another huge waste of energy.
Externally pressure fill the boiler: You can bring the model to the bank, connect up an external tank and pump and fill the boiler under pressure. This is a bit of a messy process but a huge improvement over filling by gravity. It saves venting off the pressure but you have to connect and disconnect the filling pump every time.
On board tank with manual pump: This has the advantage of having the pump permanently connected so all you have to do is open the filling valve and operate the pump. You then have additional piping and the feed tank to incorporate into the build but the feed tank doen't have to be filled until you want to fill the boiler so it shouldn't have to effect displacement.
On board tank with engine driven pump: Saves having to pump manually but the three way valve can only be manually set so differences in model operation can ultimately affect the boiler level. The pump does only run when the engine is running so there is a degree of control there and the idea being if you get the position of the three way valve just right it will fil the boiler as the water is being used by the engine. More complex piping and more expense in buying the pump as well as limiting the engine choice to the ones that can have a feed pump attached. The feed tank must also be filled so there are displacement considerations and the pump is taking power from the engine so there could be a slight loss of performance.
On board tank with engine driven pump and automatic feed valve: The ultimate in feed systems in so far as it can be left to do the job automatically. The pump runs with the engine, or can even be electrically driven, and the three way valve position is controlled by a level sensor on the boiler. Expensive and complicated and the feed tank must be filled but the most efficient as far as the boiler operation is concerned.
For myself I went for an on board tank with a manual pump. To me this is a reasonable compromise of complexity and convenience and suits the operation I envisage of bringing the boat into the bank every half an hour to fill the lubricator, empty the condenser, fill the boiler and fill the gas tank. If you don't go for an on board tank I would recommend that you use an external tank and pump rather than have to go through the wastefull process of venting off the boiler and having to raise pressure again.
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